1. Field of the Invention
The present invention relates to a light guide plate and a surface light source using the same, and especially to a light guide plate providing highly uniform illumination.
2. Description of Related Art
Recently, color liquid crystal display devices have been widely used in various applications, such as in portable personal computers, liquid crystal display televisions, video built-in type liquid crystal televisions, etc. This kind of liquid crystal display device comprises a back light unit and a liquid crystal panel. An under-lighting system in which a light source is disposed just under the liquid crystal panel, or an edge-lighting system in which a light source is disposed on a side surface of a light guide plate, is used as the back light unit.
Recently, edge-lighting systems have been more frequently used because they allow a size of the liquid crystal display device to be reduced. In the edge-lighting system, the light source is disposed at a side surface of the light guide plate, and the light guide plate uniformly emits light from an emission surface, which illuminates the liquid crystal panel. Thus, an edge-lighting system is also called a “surface light source”.
In such a surface light source, the light guide plate is formed from a planar transparent member such as an acrylic resin plate or the like. Light beams emitted from a light source, such as a cold cathode fluorescent lamp (CCFL) or a light emitting diode (LED), are transmitted through a side surface (light incidence surface) into the light guide plate. The incident light beams are totally internally reflected in the light guide plate between a light emission surface and a bottom surface of the light guide plate, and then transmit out through the light guide plate. A plurality of light reflection dots having a light scattering function are formed on the bottom surface to increase the uniformity of illumination provided by the light guide plate.
A conventional surface light source 10 as shown in FIG. 5 has a light source 110 and a light guide plate 12 having a bottom surface 120 and a light incidence surface 140 adjacent to the light source 110. Because the light source 110 is a linear light source, the illumination it provides is not uniform, being highest at a center portion of the light source 110, and being lower at the two ends. As a result, the light guide plate 12 has dark regions 130 on two corners of the bottom surface 120. Thus, the illumination provided through the light emitting surface (not shown) is non-uniform and a high-quality display image cannot be obtained in a liquid crystal display using the surface light source 10.
FIG. 6 shows another conventional surface light source 20. The surface light source 20 includes a plurality of LEDS 210 used as light sources, and a light guide plate 22 having a light incidence surface 240 and a bottom surface 220. The LEDs 210 emit light beams into the light guide plate 22 within an incident angle range of about 130 degrees, so that a plurality of areas outside the light incident angle range of the LEDs 210 become dark regions 230, since fewer light beams enter thereinto. The illumination provided by the light emitting surface (not shown) is also non-uniform.
In order to solve the above problems, various proposals have been made, including increasing the number of LEDs or increasing a distance between the light source and the light guide plate. Although illumination uniformity can be achieved, a cost of the surface light source is increased or a size of the surface light source is increased.